1. Field of the Invention
The present invention relates to an image forming apparatus, which manages light intensity of light beams emitted by an optical writing unit such as a multi-beam writing unit when image forming line speed is switched, an optical writing control method employed by the image forming apparatus, and a program for implementing optical writing control for an image forming apparatus.
2. Description of the Background Art
Image forming apparatuses employing electrophotography have an optical writing unit, an image-bearing member (hereinafter, referred to as a “photoconductor”), and a development unit, and form an image on a recording medium. Specifically, a latent image is formed on the photoconductor by directing a light beam onto the photoconductor using the optical writing unit, with the light beam representing image data. The latent image is developed as a toner image by the development unit, and then transferred onto the recording medium. In such image forming apparatuses, the optical writing unit has a light source, such as a laser diode (LD), which can be controlled to an activated condition at a given timing to write a latent image corresponding to the image data.
In such image forming apparatuses, the image forming line speed may be changed depending on the type of recording medium. For example, an image forming line speed for an overhead projector (OHP) sheet or thick paper may be set slower than an image forming line speed for plain paper when an image forming operation is conducted. Specifically, when an image forming operation is conducted for OHP sheet or thick paper, the image forming line speed is set to a slower speed in order to allow the toner to melt effectively, so that no reflection appears at the boundaries of toner particles. With such configuration, image color haziness can be prevented and thereby vivid images can be formed. Further, setting a slower image forming line speed can enhance image glossiness.
JP-2002-166592-A discloses an image forming apparatus that includes such a configuration for adjusting the image forming line speed. More specifically, JP-2002-166592-A discloses an image forming apparatus including a multi-beam optical system to write a latent image on an image bearing member, in which two or more light sources are used to emit a plurality of laser beams to write latent images on the image bearing member. In addition, the image forming apparatus includes a writing controller and LD driver to switch the numbers of beams used by the multi-beam optical system. The multi-beam optical system may switch or change the numbers of beams used for image writing depending on the image forming mode, defined by factors such as image density level, switching of transport speed depending on types of transfer member, and so forth, and including speed mode/image quality mode, plain paper mode/special paper mode, and/or full-color mode/black-and-white mode. With such a configuration, image forming can be conducted under various types of image forming modes without posing an undue burden on the polygon mirror motor that is the heart of the system.
In JP-2002-166592-A, the number of beams (hereinafter “beam number”) used by the multi-beam scanning system or unit can be switched or changed as follows. When the beam number is to be decreased, the number of laser diodes (LDs) to be set at activated condition is decreased (i.e., the number of deactivated LDs is increased), and when the beam number is to be increased, the number of LDs to be set at activated condition is increased. When the beam number is to be increased, a light beam detector detects a newly increased light beam to set a main scanning line synchronization signal for the newly increased light beam. In such detection process, one line scanning operation is required while maintaining an activated condition for LD, which is newly used as a light source due the beam number increase.
However, if one line scanning operation is conducted, a light beam scans for one line on a photoconductor, by which a toner image may be formed unnecessarily. Further, when a LD is shifted from a deactivated condition to an activated condition, the LD may need more time than one line scanning operation to set a light intensity of LD at a required level, by which the toner image may be further formed unnecessarily. Further, when the beam number is switched or changed, various control processes may be required, by which more time may be needed for setting the required beam number, and thereby throughput performance of the apparatus, such as print speed, may deteriorate to slower speed. When an image forming operation is conducted, neither such unnecessary toner image formation nor the deterioration in print speed is desirable.